This invention relates to a process for etching a metal-containing layer on a semiconductor substrate.
Referring to FIGS. 1a and 1b, the present process is used to etch a stacked metal-containing layer 15 on a semiconductor substrate 10, for example, a silicon or gallium arsenide wafer. The metal-containing layer 15 typically comprises a diffusion barrier and/or adhesion promoting layer 20, such as Ti, TiN, Ta, TaN, W, WN, and the like, a metal layer 25 of aluminum, copper, tungsten, or their alloys with each other and/or other materials, and an anti-reflective layer 30, such as TiN, silicon oxynitride, or an organic anti-reflective material. Metal interconnect lines 32 are formed by etching the stacked metal layer 15 to electrically connect the active devices on the substrate 10. A typical process sequence for forming the metal interconnect lines 32 comprises the steps of (1) sequentially depositing each layer 20, 25, 30 of the metal-containing layer 15 on a substrate 10, (2) formation of a mask layer 35 that captures a pattern that is to be transferred into the metal-containing layer 15, and is typically composed of photoresist, but can be made of other materials, such as silicon dioxide or silicon nitride, (3) etching the metal-containing layer 15 to transfer the pattern captured in the mask into the metal-containing layer 15 to form the interconnect lines 32, (4) ashing with oxygen-containing plasma to remove (or strip) any remaining resist (if any is present in the mask) and to passivate metal-containing lines by removing residual etching species to prevent corrosion, (5) depositing a dielectric layer (not shown) to isolate the metal interconnect lines 32 from the next level of metal interconnect lines and/or the environment, (6) additional sequences of process steps to form conductive metal studs (not shown) in the dielectric above the metal interconnect lines 32 to connect them to the lines in the next metal layer, and (7) planarization of the dielectric layer that can be a result of dielectric deposition process or can be performed by chemical mechanical polishing (CMP) after the studs are formed. The present invention relates to etching step (3) in this sequence in which the pattern of lines or other features captured in the photoresist or other mask layer is transferred into the metal-containing layer 15 by a plasma etch process (sometimes referred to as reactive ion etching or RIE).
As the semiconductor industry strives to build cheaper and faster devices, it has to increase surface density of the devices on the semiconductor substrate 10 while trying to keep the conductivity of the metal interconnects as high as possible. As a result, with each device generation the smallest in-plane dimensions of the interconnect lines 32 (also known as critical dimension or CD) are scaled down faster than the stacked metal layer thickness. At present, it is not uncommon to see interconnect lines 32 with the aspect ratio (which is the ratio of line height to its width) as high as two or three, and in the near future it may be as high as four. This poses especially stringent requirements on the etch process.
To fabricate such high aspect ratio interconnect lines 32, it is necessary to perform highly anisotropic etching of metal-containing layer 15. FIG. 2a illustrates isotropic etching in which the etch rates in the direction parallel to the plane of the substrate 10 (into the side-wall) are substantially the same as the etch rates that proceed vertically (so that the distance a is the same as the distance b). This results in undercutting below the mask layer 35 that makes it difficult to etch spaces between the interconnect lines 32 that are narrower than twice the thickness of the etched depth, which means that only an aspect ratio (for the line spacing) of less than 0.5 can be achieved. FIGS. 2b through 2d show anisotropic etching processes. FIG. 2b shows etching still proceeding into the sidewall but at a slower rate than etching in the vertical direction (a&lt;b). The most desirable case of highly anisotropic etching is shown in FIG. 2c, when etch rate in the direction parallel to the substrate is 10 exactly zero (a=0). FIG. 2d illustrates the case of highly anisotropic etch, when the bottom of an etched line is wider than its top, or in other words etch rate in the parallel direction is negative (a&lt;0) and the profile angle .alpha. is more than 90.degree.. Though, all of these situations are possible while etching metals and alloys such as aluminum, copper, tungsten, titanium, tantalum, etc., the shape of the etched feature shown in FIG. 2c is the most desirable because it allows, at least in principle, a spacing between metal interconnect lines 32 of very high aspect ratios.
The highly anisotropic etch achieved today is performed in a plasma etching apparatus. Plasma provides anisotropic etching because it possesses a highly anisotropic source of energy-ions. The ions present in the plasma are accelerated towards the substrate 10 in the plasma sheath, and collisions of these ions (X.sup.+, FIG. 3) with the surfaces parallel to the substrate provide additional energy (in excess of the thermal energy) which accelerates certain surface reactions. Unlike the ions, neutral species (Y.degree.) are not directional and, therefore collide with all the surfaces exposed to plasma. The thermal energy available from the surface and the neutral plasma species does not differentiate between the surface orientation. Thus, if the set of surface reactions responsible for etching is not sensitive to the additional energy provided by the ions, as is the case for etching many metals with halogens, such as etching aluminum with chlorine or etching tungsten with fluorine (in the absence of contaminants), isotropic etching is obtained. When, on the other hand, the etching reaction has an activation energy that is higher than the thermal energy, it only will take place on those substrate surfaces that are subjected to the energetic plasma ion bombardment, and etching proceeds essentially in the direction perpendicular to the substrate 10.
To facilitate etching, the etching process gases include reactive etching gases that easily react with the material being etched to form volatile gaseous byproducts which are removed from the reactor with a vacuum pump. For example, it is known that halogen gases react with many metals to form volatile metal halides. However, most metals (such as aluminum which is currently used as an interconnect material and copper that is expected to replace aluminum) spontaneously react with halogen gases. Thus highly anisotropic etching of the metal-containing layer 15 is not possible in the absence of reactants other than halogen gases. To achieve anisotropy, a gas inhibitor or passivator that forms an inhibitor layer 40 deposited on sidewalls of the freshly etched metal features is added to the etching gas. The inhibitor layer 40 partially or completely blocks the access of the etching gas (usually halogen) to the sidewall to provide anisotropic etch. At the same time, it does not accumulate on the surfaces subjected to the ion bombardment, as it is being sputtered or etched off with the ion assistance, thus allowing the etching process to proceed. Thus the gas inhibitor has two somewhat conflicting requirements, it has to be deposited easily on the sidewalls and form a dense layer impermeable to etch gas, and it has to be easily etchable under ion bombardment in the atmosphere of the same etch gas. These requirements make finding a good inhibitor gas difficult, and at the same time, it is essential for successful profile etching of metal interconnect lines 32.
In conventional chlorine-based etch of aluminum-containing interconnects, organic photoresist, typically used as a mask material, is etched away (eroded) at a rate that is typically around 0.2 to 0.5 times the etch rate of the aluminum-containing layer. It is believed that the byproducts of this photoresist erosion serve as a passivator gas. There are two reasons to think that this is the case. It is well known that aluminum etch process conditions can be adjusted to change the etching selectivity (relative etch rate) of aluminum to photoresist. It is known that by reducing the etching selectivity or by sacrificing more of the photoresist material, etching becomes more anisotropic (since the protective action of the inhibitor is reduced on smaller, high aspect ratio features, the increased photoresist erosion allows anisotropic etching of smaller features). Also, a compositional analysis of the sidewall polymer reported in an article by P. Czuprynski, O. Joubert, L. Vallier, M. Puttock and M. Heitzmann, J. Vac. Sci. Technol. B, 16(1), (1997), 147, demonstrated that the sidewall polymer contains a large amount of carbon--the main atomic component of the photoresist. Since their etch gases included only Cl.sub.2 and BCl.sub.3, the carbon could originate only from the photoresist that is primarily composed of carbon and hydrogen. Thus, in a typical aluminum etch processes, the byproducts of photoresist etching process are an integral part of the etching chemistry and provide anisotropic etching of the metal-containing layers 15.
As the critical dimensions of the etched features continue to shrink, it becomes more difficult to obtain anisotropic etching or to control etch profiles by conventional methods that rely on photoresist erosion. Smaller critical dimensions and larger aspect ratios require more inhibitor species be present in the plasma for profile control because it is more difficult for the inhibitor to penetrate into the narrower spaces between the etched metal interconnect lines 32. However, less photoresist is available because conventional lithography methods can produce small features only if the photoresist layer is sufficiently thin. There is often not enough photoresist material in the thin photoresist mask layer 35 to provide sufficient carbon species that deposit and control the etch profile. Also, it is often beneficial to use mask materials other than organic photoresist, such as silicon dioxide or silicon nitride (also called hard masks). These materials have the advantage of not being easily etched in a chlorine-containing plasma which is a more commonly used for etching aluminum-containing layers. Thus modern etching processes often fail to provide sufficient inhibitor species to anisotropically etch features in the metal layers.
Thus there is a need for an etching process that provides anisotropic etching of metal layers that is not dependent upon the thickness or composition of the photoresist layer on the substrate. There is a further need for an etching process that provides highly anisotropic etching of metal features, even if the etched features have high aspect ratios and small critical dimensions. There is especially need for a process that provides anisotropic etching of metal layers that are etched with inorganic hard masks with thin overlying photoresist layers.